Hydrophilic coatings are advantageous for certain coating applications, such as where coated surfaces exhibiting anti-fouling, easy-to-clean, self-cleaning, and/or anti-fogging properties are desired. Such coatings can be particularly useful, by way of example, for application to surfaces exposed to the outdoor environment. Building structures as well as other articles that are exposed to the outdoors are likely to come in contact with various contaminants, such as dirt, oil, dust, clay, among others. Rainfall, for example, can be laden with such contaminants. A surface with a hydrophilic coating deposited thereon may be anti-fouling by preventing contaminants in rainwater from adhering to the surface when the rainwater flows down along the coated surface. Moreover, during fair weather, air-born contaminants may come in contact with and adhere to surfaces. A surface with a hydrophilic coating deposited thereon may be self-cleaning because the coating has the ability to wash those contaminants away when the surface comes in contact with water, such as during a rainfall.
Coatings having self-cleaning and/or anti-fouling properties may also be advantageous for application to surfaces that are exposed to indoor contaminants, such as, for example, kitchen contaminants, such as oil and/or fat. An article with a hydrophilic coating deposited thereon can be soaked in, wetted with, or rinsed by water to release contaminants from the coating and remove them from the surface of the article without use of a detergent.
Coatings having anti-fogging properties can be particularly useful in many applications as well. For example, articles containing surfaces where visibility is important, such as windshields, windowpanes, eyeglass lenses, mirrors, and other similar articles can benefit from a coating with anti-fogging properties because they contain surfaces that can often be fogged by steam or moisture condensate or blurred by water droplets adhering to the surface thereof. It is known that the fogging of a surface of an article results when the surface is held at a temperature lower than the dew point of the ambient atmosphere, thereby causing condensation moisture in the ambient air to occur and form moisture condensate at the surface of the article. If the condensate particles are sufficiently small, so that the diameter thereof is about one half of the wavelength of the visible light, the particles can cause scattering of light, whereby the surface becomes apparently opaque, causing a loss of visibility.
A surface with a hydrophilic coating deposited thereon may be anti-fogging because such a coating can transform condensate particles to a relatively uniform film of water, without forming discrete water droplets. Similarly, a surface with a hydrophilic coating deposited thereon can prevent rainfall or water splashes from forming discrete water droplets on the surface, thereby improving visibility through windows, mirrors, and/or eyewear.
In view of these and other advantages, various hydrophilic coating compositions have been proposed. Some of these coatings achieve their hydrophilicity through the action of a photocatalytic material that is dispersed in a silicon-containing binder. For example, U.S. Pat. No. 5,755,867 (“the '867 patent”) discloses a particulate photocatalyst dispersed in a coat-forming element. The photocatalyst preferably consists of titanium dioxide particles having a mean particle size of 0.1 micron or less, where the titanium dioxide may be used in either the anatase or rutile form. The coat-forming element is capable of forming a coating of silicone resin when cured and is comprised of an organopolysiloxane formed by partial hydrolysis of hydrolyzable silanes followed by polycondensation. In the '867 patent, the silicone coat-forming element in itself is considerably hydrophobic, but upon excitation of the dispersed photocatalyst, a high degree of water affinity, i.e., hydrophilicity, can be imparted to the coating.
Japanese Patent Application JP-8-164334A discloses a coating film-forming composition comprising titanium oxide having an average particle diameter of 1 to 500 nanometers, a hydrolyzate of a hydrolyzable silicon compound, and a solvent. The hydrolyzable silicon compound is an alkyl silicate condensate expressed by the formula SinOn−1(OR)2n+2 (where n is 2 to 6, and R is a C1-C4 alkyl group). The hydrolyzate results from hydrolyzing the alkyl silicate condensate at a hydrolysis percentage of 50 to 1500%. The hydrolyzate itself, however, is not believed to be hydrophilic.
U.S. Pat. Nos. 6,013,372 and 6,090,489 disclose photocatalytic coating compositions wherein particles of a photocatalyst are dispersed in a film-forming element of uncured or partially cured silicone (organopolysiloxane) or a precursor thereof. The photocatalyst may include particles of a metal oxide, such as the anatase and rutile forms of titanium dioxide. According to these patents, the coating compositions are applied on the surface of a substrate and the film-forming element is then subjected to curing. Then, upon photoexcitation of the photocatalyst, the organic groups bonded to the silicon atoms of the silicone molecules of the film-forming element are substituted with hydroxyl groups under the photocatalytic action of the photocatalyst. The surface of the photocatalytic coating is thereby “superhydrophilified,” i.e., the surface is rendered highly hydrophilic to the degree that the contact angle with water becomes less than about 10°. In these patents, however, the film-forming element in itself is considerably hydrophobic, i.e., the initial contact angle with water is greater than 50°. As a result, the coating is initially hydrophobic, and it is only upon excitation of the dispersed photocatalyst that hydrophilicity is imparted to the coating.
U.S. Pat. No. 6,165,256 discloses compositions that can hydrophilify the surface of a member to impart anti-fogging properties to the surface of the member. The compositions disclosed in this patent comprise (a) photocatalytic particles of a metallic oxide, (b) a precursor capable of forming a silicone resin film or a precursor capable of forming a silica film, and (c) a solvent, such as water, organic solvents, and mixtures thereof. The solvent is added in an amount such that the total content of the photocatalytic particle and the solid matter of the precursor in the composition is 0.01 to 5% by weight. As in the previous examples, however, hydrophilification of the coating takes place upon photoexcitation of the photocatalyst, while the film forming material in itself is not hydrophilic. Therefore, such a coating is not believed to be hydrophilic prior to excitation of the photocatalyst.
These prior art hydrophilic coatings that achieve their hydrophilicity through the action of a photocatalytic material that is dispersed in a silicon-containing binder suffer, however, from some drawbacks. One notable drawback is that these coatings do not exhibit hydrophilicity until photoexcitation of the photocatalyst. In addition, these compositions typically require a forced cure, i.e., they cannot be efficiently cured at room temperatures.
U.S. Pat. No. 6,303,229 (“the '229 patent”) discloses another composition that may include a photocatalytic material. The coating disclosed in the '229 patent is formed from applying a coating composition that contains a silicone resin, which acts as the binder and which is obtained by hydrolyzing and condensation-polymerizing certain tetra-functional alkoxysilanes. The coating compositions may also include colloidal silica. Evidently, the coated film formed from the composition disclosed in the '229 patent can be initially hydrophilic. This initial hydrophilicity, however, is believed to result from the presence of colloidal silica in the composition rather than the silicone resin itself. It is believed that the silicone resin disclosed in the '229 patent is not itself hydrophilic because the desired pH of the resin is 3.8 to 6.0, which indicates that Si—OR groups exist in the resin in an amount to prevent gellation and hydrophilicity. As a result, such a coating suffers from some drawbacks. For example, such a coating that includes a photocatalytic material cannot be effectively applied directly over an organic base material without the inclusion of a primer layer between the substrate and the coating composition containing the silicon resin.
It would be advantageous to provide multi-layer coatings that include a coating layer that comprises a liquid composition comprising photocatalytic material and a binder, wherein the multi-layer coating is hydrophilic prior to excitation of the photocatalyst and can be rendered super-hydrophilic upon excitation of the photocatalyst.